Evolution of Genes
Individual genes and whole genomes change over time. Indeed, evolution of genes ultimately accounts for the evolution of organisms that is seen in the fossil record: Humans evolved from earlier apes, and those creatures from their ancestors, by gene changes in the earlier creatures. Just as the fossil record can be examined to understand the patterns of organismic evolution, so too can genes be compared to understand genomic evolution.
Natural Selection
Most changes that occur in genes are subject to natural selection, the process first outlined by Charles Darwin in 1859. In natural selection, a heritable change arises by chance. If the organism with that change is better able to survive and reproduce, it will leave more descendants in future generations. These descendants will also carry the new genetic change, and as they reproduce, the change will become more widespread in the population. On the other hand, if the change decreases an organism's survival rate, it will be lost from the population. It is also possible to have a neutral change, with no immediate effect on survival. Such "hidden" genetic variation within a population provides grist for evolution when it offers a selective advantage under new environmental conditions.
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